Many types of modem machinery today include rotational mechanical members used to perform various tasks. For example, in the manufacturing setting, rotational mechanical members are used as robotic arms to manipulate workpiece material or to position manufacturing tools such as drills and welders in relation to workpieces. In addition to use in manufacturing, rotational mechanical members are also used in a wide variety of other applications, such as angle positioners for view finding equipment and angle selectors for artillery firearms.
Important to most of these applications, is the accurate determination of the angle position of the mechanical members. This angle position information is used in determining the overall position of the mechanical member and its positional relationship with respect to either workpieces or targets. To determine angle position, many applications use resolver devices that are connected to the rotational end of the mechanical members and provide angle information concerning the rotational position of the mechanical members. A common resolver device used for these applications is an Inductosyn.TM. resolver manufactured and distributed by Farrand Controls located in Valhalla, N.Y.
A typical resolver, such as the Inductosyn.TM. resolver, contains a series of electrical coils that are used to determine the angle position of a rotational mechanical member or shaft to which the resolver is connected. Specifically, the resolver includes a set of first and second output windings (also known as cosine and sine windings) and an input or reference winding that is spaced apart from the first and second output windings. Importantly, the position of the first and second output windings relative to the input winding defines the angle position of the shaft.
To determine the angle position of the shaft, a continuous sinusoidal voltage is typically input into the input winding. Through electrical induction, the voltage from the input winding is transferred to the first and second output windings, which, in turn, generate first and second output signals. The first and second output signals generally represent the cosine and sine of the angle of the rotational mechanical member. To determine the angle, these two signals are analyzed using a resolver-to-digital converter (RDC) or similar angle determination circuit that performs a calculation on the signals and in a closed loop system increments or decrements a counter register indicating the current angle measurement of the mechanical member.
Although resolvers typically provide accurate angle information, there are some drawbacks to their use and also to the precision to which the angle measurement is determined. Specifically, typical resolvers such as Inductosyn.TM. resolvers use air core transformer designs. These air core transformer designs produce low inductance coupling between the input winding and the first and second output windings. As such, the resolver experiences unacceptable power loss from the input windings to the output windings. This power loss typically causes several problems with implementation of the resolver in terms of efficiency, heating, impedance matching, and introduction of noise.
For instance, a typical air core transformer generally used in resolvers has a coupling ratio from the input coil to the output coils of 150-200:1. As such, for every volt input into the resolver the output windings output a voltage in the range of 0.006 to 0.005 volts. This poor coupling between the input and output windings is compensated for in most resolver systems by the use of gain amplifiers that are connected to the output windings for gaining the first and second output signals prior to input in the RDC or similar angle determining circuit for determining the angle measurement. Although the gain amplifiers compensate for the low coupling ratio of the resolver, the use of gain amplifiers in the resolver system does have some drawbacks and limitations.
Specifically, in typical electronic applications there must be proper impedance matching between different electrical components to achieve desired performance. However, the impedance output of gain amplifiers changes with the amount of amplification the amplifiers have been set to amplify. Impedance matching problems become troublesome when the gain amplifiers are set to large gains. As such, in resolver systems that require large magnitude gain amplifiers, the RDC or similar angle determining circuit and the gain amplifiers of the resolver system must be adjusted on a system by system basis to ensure that proper impedance matching is achieved. This can be time consuming during manufacture of the resolver systems and also in later recalibration of the systems. Further, because the amplifiers and the RDC circuit have to be impedance matched, conventional revolver systems do not allow several resolvers to be multiplexed with one RDC or similar type angle determining circuit.
Additionally, gain amplifiers are typically susceptible to the introduction of signal noise that can affect the resolution of the angle measurement. Noise such as input capacitance and electrical capacitance from spurious signal sources, (e.g., 60 Hz power source), can be introduced into the data signals from the output windings of the resolver. Due to the large magnitude of gain supplied by the gain amplifiers, the electrical noise present on the input of the amplifier is significantly amplified. This amplification of the signal noise further causes the relatively small magnitude electrical data signals from the output windings of the resolver to be obscured by the larger magnitude electrical noise.
Noise problems associated with the amplifiers may be exacerbated by the use of continuous input signals and the continuous connection of the output windings of the resolver to the RDC or similar angle determining circuit. Specifically, the continuous connection of the resolver to the RDC allows all electrical noise to be introduced into the output signals.
In addition to the amplification problems associated with the poor coupling of the input and output windings, there are also problems associated with the power ratings of the resolvers. Specifically, the typical reference coil of a resolver has an impedance that is mostly resistive and typically on the order of 2 ohms or less. Because of the low resistance of the input coil, the input drive signal power can exceed 2 watts with only an input of 2 volts (e.g., power=volt.sup.2 /resistance). The problems associated with the input power characteristics are exacerbated by the fact that typical resolver systems use a continuous sinusoidal input signal for angle measurement. This continuous signal exerts a constant signal on the resolver that can damage the resolver if the amplitude of the input signal is relatively large. As such, due to the low power rating and the continuous input signal, a lower voltage input signal must be used in conventional resolver systems to ensure that the resolver does not experience failure.
The requirement for a lower voltage has a two-fold effect. First, as stated earlier, due to the poor coupling problems of the resolver, a higher voltage input is desired, but is limited by potential for failure of the resolver at these high voltages. A second additional problem is that typical power supplies used to power the machinery surrounding the resolvers and thus, typically used as power supplies for the resolvers are 12 and 24 volt supplies, which is well above the power rating limits for continuous voltage input of the resolvers. As such, in implementation, the input signal supplied by the conventional 12 and 24 volt supplies must be reduced before input into the resolver. The reduction of the signal input creates a power loss that can be 4 to 6 times the power required for the device.
An additional concern connected with the poor coupling of the input and output windings of the resolver is the overall energy consumption of each resolver. This is especially troublesome in situations where the power supplied to the resolvers is a temporary energy source such as a battery pack. In these situations, the continuous analog input signal used by typical resolver systems will significantly reduce the lifetime of the energy source. At least one resolver system has attempted to conserve energy consumed by the resolvers in situations in which the resolver is operating on auxiliary or batter power. Specifically, this resolver system uses a trapezoidal input signal that approximates the analog sinusoidal waveform typically used as an input to the resolver. The resolver system also uses a reduced sampling rate for sampling the output signals from the output windings. The use of the trapezoidal input signal and the reduced sampling rate allegedly conserves the energy consumed by the resolver system.
However, the use of the trapezoidal input drive signal does have some drawbacks that may be unacceptable in resolver system applications. Specifically, the trapezoidal waveform supplies an almost continuous input signal to the resolver so as to approximate the usual analog input signal. As such, not only does the trapezoidal signal consume energy because of its almost continuous signal characteristics, the trapezoidal input signal may also cause damage to the resolver, because of the power rating characteristics of the resolvers. Additionally, this resolver system does not receive precise angle measurements when utilizing the trapezoidal signal. Specifically, the trapezoidal signal may saturate the gain amplifiers making precise measurement of the signals output by the output windings unobtainable. In this instance, the resolver system can only determine whether the output signals are positive or negative. This at best allows the system to only resolve the quadrant of the angle of the shaft and not a precise angle measurement.
An additional problem associated with conventional resolver systems is the use of individual drive circuits, amplifiers, and RDC circuits for each resolver in a manufacturing application. Individual components are generally used because the resolver systems use continuous analog inputs and also require individual gain and impedance matching for each amplifier and RDC circuit or similar type angle determining circuit. The use of individual components for each resolver can become quite costly and also add to the overall size of machinery. Further, because of the high gains, off-the-shelf or legacy components may not operate in the resolver system, thereby requiring specifically designed components. Additionally, many movable mechanical members of machinery have multiple shafts each having several axis of rotation. As such, each axis of rotation may require a resolver. Thus, the number of resolvers for each moveable mechanical member may become large, and having individual, specialized components for each resolver will add to the cost and size of the machinery.